1. Field of the Invention
This invention relates to digital receivers, and more particularly to digital receiving methods and systems that are intended to be used to receive signals within multiple wavebands that are separated from each other along the electromagnetic spectrum.
2. Description of the Related Art
Conventional automobile radio systems have amplitude modulation (AM) and frequency modulation (FM) capabilities. Many automobiles are also equipped with cellular radios for both reception and transmission. Additional services that utilize other portions of the electromagnetic spectrum and are either currently being implemented or are expected to be implemented in the future include facsimile, computer, and global positioning system (GPS) services. Reception of the various services is complicated by a desire to accommodate multiple simultaneous users of either different services, or of the same service. For example, it is desirable to have a capacity for cellular telephone calls at the same time the radio is playing on either AM or FM. It is also anticipated that different users will be able to plug into the communications system at the same time, such as by separate headphones assigned to different passengers, with each passenger having the ability to receive his or her own radio station while the other passengers are listening to their individual stations.
The different broadcast bands are very dissimilar in terms of their bandwidths, modulation technology and band operation. The conventional approach to receiving multiple channels over multiple bands is to simply provide multiple receivers, with a separate receiver assigned to each band and multiple receivers assigned to the same band if it is desired to permit multiple simultaneous uses of that band. Each additional receiver incurs a penalty in terms of cost, weight, power and space requirements.
Digital receivers have been perceived as a way to accommodate very dissimilar types of modulations with a single receiver mechanism, eliminating the need for costly multiple receivers. Since the channel selection frequency tuning, channel isolation and de-modulation are all accomplished digitally, only a single digital receiver path is required for all of these functions. A transition between different broadcasting formats and bandwidths is accomplished by simply changing filter coefficients in the digital filters and the de-modulation algorithms in a programmable de-modulator. Such a system is envisioned in copending U.S. patent application Ser. No. 07/293,894, filed Jan. 5, 1989 by Stone et al. and assigned to Hughes Aircraft Company, the assignee of the present invention.
While the co-pending patent application achieves a significant reduction in system complexity and cost by using common digital computation for the different service bands, it is still costly when used for services that are widely separated in frequency. For example, AM uses the broadcast band of 0.540-1.600 MHz with 15 KHz channel bandwidths, FM has a broadcast band of 87.9-107.9 MHz with 400 KHz channel spacings and 150 KHz channel bandwidths, while cellular phone occupies the broadcast band of 865-895 MHz with 24 KHz channel bandwidths and 30 KHz channel spacings (with either an FM or digital modulation broadcasting format). It would be very expensive, and beyond the limits of currently available equipment, to digitize the entire electromagnetic spectrum over these different bands with a single analog-to-digital converter (ADC). However, if the three bands are digitized separately with separate ADCs for each band, the system expense is increased considerably because of the multiplicity in ADCs.
In U.S. Pat. No. 4,884,265 to Schroeder et al., assigned to Loral Corporation, a frequency division multiplex input signal is also sampled in an ADC. The samples are translated by mixing with baseband frequency signals to yield real and imaginary values corresponding to phase information in the original modulation signals. After translation, the samples are filtered in real and imaginary digital filters. The original modulation information is then recovered by analysis of the positions of vectors in the complex plane represented by the real and imaginary values. The translation is preferably performed by multiplying the input samples by digital values which correspond to sine and cosine values of local oscillator signals at baseband frequencies. The use of pre-select filtering prior to translation to decimate the input samples and thereby reduce subsequent processing requirements is suggested.
While it is an improvement in digital receiver design, the de-modulator disclosed in this patent again does not resolve the problem of handling multiple simultaneous uses of widely separated bands without unduly increasing the complexity or cost of the necessary ADCs.